If you are having the same problem as the OP (ie the touchscreen works with the S-Pen but not your fingers), don't take your phone apart. Just go to application manager (in your settings) - find Google Play Store and uninstall all the updates. Then restart your phone and the touchscreen will work again.
If it doesn't (but it will in 99% of cases), uninstall updates for other Google / Samsung apps.
Then restart the phone.